The invention relates to control of the neutralization of surface charges existing on objects such as paper, fabrics, plastic sheets or the like.
It is well known to remove electrostatic charges from paper, fabrics, plastic sheets or the like by means of a brush which consists of thin conductive fibers, e.g., high-grade steel or carbon fibers, and neutralizes the charges. There are two methods utilized when employing such conductive fiber brush. The so-called passive discharge is based on the phenomenon that the breakdown field strength is exceeded and the ionized air molecules effect compensation of the surface charges. As a result, electric discharge current flows from the surface via the brush to ground. The active discharge method is based on corona discharge, the charge carriers being produced by applying high voltage to the fiber brush. Costly high-voltage supply, however, and ozone formation due to corona discharge are disadvantages of this latter method.
Neutralization of surface charges is effected by means of stationarily mounted brushes which remove the charges by mechanical contact with the surface. In DE-OS 29 50 192 A1, neutralization of electrostatic surface charges is described, the mechanical stress of the surface being kept as small as possible. Direct contact between surface and neutralizing brush is avoided by mounting an additional, non-conductive spacer element keeping the brush at a small distance (a few micrometers) from the surface. This method is based on the so-called corona discharge phenomenon according to which during the approach of a thin conductive wire towards a highly charged body an appropriate electrical field is produced on the periphery of the wire in order to effect ionization of a gas whereby the charged body is discharged. The above space elements prevent the brush from being soiled which would entail a reduction of the discharge properties of the brush. It is, however, a disadvantage that brush and object must be kept at a constant distance relative to each other throughout the process, and complete neutralization of the charge is achieved only when a certain charge condition exists.
U.S. Pat. No. 4,363,070 discloses a method in which the surface charges are neutralized without any contact between brush and object. A neutralizing brush is positioned at a distance of about 50 .mu.m from the surface of the object and connected to an AC voltage source. The voltage source applies AC voltages across the brush and the grounded guide roller which vary both in frequency and amplitude. A charge sensor detects the actual surface charge condition which is compared with the nominal condition, a controlled variable being deduced therefrom and controlling the potential and/or the frequency of the AC voltage source so as to achieve complete neutralization of the surface. It is a disadvantage of this method that a high-voltage AC power supply is required and that it cannot react to thickness variations and/or to changes in the material properties of the object. When thickness or material properties of the object are changed, the brush position must be adjusted manually.